The turbine arrangement according to the above thus comprises both a gas turbine unit and a steam turbine. Such a turbine arrangement is often called “Combined cycle” (CC). Since the above-mentioned first unit has two gas flow paths, it is often called a “split stream boiler” (SSB).
Different kinds of CC turbine arrangements have been suggested in order to achieve a high efficiency of the arrangement. In order to achieve a high efficiency in the steam cycle, the temperature of the gas from the gas turbine unit, which gas is used to heat the steam in the steam cycle, ought to be as high as possible. This may be achieved by having a high combustion temperature in the combustion chamber from which gas is supplied to the gas turbine expander. However, such a high combustion temperature leads to certain disadvantages. Such disadvantages are for example that the formation of undesired nitrogen oxides may increase, that improved cooling of the gas turbine expander is needed, that maintenance costs increase or that the useful life is reduced.
A known manner of increasing the temperature of the gas from the gas turbine expander is to arrange a supplementary combustion unit after the gas turbine expander in order to heat the gas which is used for heating steam in the steam cycle. It is also known that such a supplementary combustion unit may be arranged in only a part of the gas flow path, i.e. an SSB is used. U.S. Pat. No. 5,628,183 describes different kinds of turbine arrangements of the kind which has been described in the first paragraph above. The document describes how an improved efficiency may be achieved by using an SSB in a CC turbine arrangement.